Power over Ethernet (PoE) technology is designed to power devices connected to a multi-channel Ethernet through an Ethernet cable, thus eliminating the need of numerous power plugs for devices such as Web cams, Ip phones, Bluethooth access points, Wlan access points, etc. This technology is standardized by the IEEE802.3af specifications. A Power Sourcing Equipment (PSE) is a device that supplies power through a twisted pair of insulated wires of the multi-wire Ethernet cable to a Powered Device (PD), as depicted in FIG. 1.
The main functions of a PSE are to look for a link to a PD, to detect a PD connected to the LAN, to supply power to the power supply line of the link, to monitor the level of power absorption on the link, and to cut off the supply of power to the link. Normally, the PSE shall cut off power on the link when the PD has been disconnected from the LAN.
A LAN in which a PSE is operating may include not only PDs, but also legacy sockets that are not configured to receive power over the LAN link. In order to prevent damaging legacy equipments by unduly applying a relatively high DC voltage to their LAN connection plugs, the PSE may be able to determine, for each of its power output ports, whether or not an output port is connected to a PD.
For this purpose, the IEEE 802.3af Draft requires that each PD include a “signature element”, that is a circuit across two power input connections of the PD having predefined impedance characteristics.
When the PSE is powered up, or when a new terminal is added to the LAN, the PSE performs a link interrogation routine in order to detect the presence of such a signature element. During the interrogation phase, the functional circuits of the PD (other than the signature element) are isolated from the power supply line by a switch. Upon a positive result of the interrogation, the isolating switch of the PD is closed, and the PSE begins to supply power to the remotely connected PD. The interrogation routine employs low-voltage signals in order to avoid damaging legacy equipment that may be connected to the LAN line instead of a PD.
Once the PSE has begun to supply power to a PD, it may also be able to detect when the PD is disconnected from the LAN, in order to avoid having a high DC voltage on an open line of the LAN.
For this purpose, the IEEE 802.3af Draft specifies that the PSE should sense the DC current that is absorbed by the PD or monitor its impedance. If the current drawn from a given output port of the PSE drops below a predetermined threshold, or its impedance rises above a predetermined value, for longer than a certain period of time, the PSE cut off the DC output supply voltage to that port. Such a PD disconnection detection ability overcomes the problem that could be created in the LAN by leaving a relatively high DC voltage on an open line, and reduces the risk of damaging legacy equipment in the event that it is connected in place of the disconnected PD.
The U.S. Pat. No. 6,986,071 discloses a power distribution subsystem (typically a PSE) configured to supply DC power over a LAN to client terminals connected to the LAN. The power distribution subsystem comprises a signal generator, in parallel with the DC power source, which adds a periodic, time-varying signal to the DC voltage that it supplies through the LAN.
A control unit monitors the time-varying voltage component of the PSE output, in order to determine whether a client terminal is connected to the output and to detect disconnection when it occurs. The magnitude and phase of the time-varying voltage at the PSE output are functions of the output impedance of the power distribution system itself, and of the load impedance of the client terminal receiving power from the subsystem.
If the client terminal is disconnected, the time-varying voltage level across the output impedance of the power distribution system changes sharply, enabling the system to immediately detect the disconnection.
Supplying a time-varying voltage on the DC power lines is particularly helpful for detecting disconnection of PDs being powered (that is after the relative interrogation phase has been completed), as well as changes in the operating state of the powered devices. Preferably, a controller of the power distribution subsystem measures the time-varying voltage signal across each power output port of the subsystem and controls the DC power distribution accordingly.
The previous approaches are implemented on the high side node, forcing an integrated solution to use a charge pump to generate an extra biasing level, above the master DC power supply, or they require extra components to build up a separate signal generator or a disconnection detector circuitry not integrated with the integrated PSE controller.